The present invention relates to a method and a device for fault detection or diagnosis in a knock sensor on a multi-cylinder internal combustion engine.
It is known that in order for knocking in an internal combustion engine to be regulated, the knocking must be reliably detected, with such detection ensuring that malfunctions of a knock sensor are reliably detected. Such knock sensors are assigned, for example, to each cylinder of the internal combustion engine. They emit an output signal which indicates whether or not knocking has occurred in the cylinder. Knocking involves instances of undesired combustion, which should be avoided.
International Published Patent Application No. WO 94/01041 describes a method for knock detection and for detection of faults in knock sensors. In this conventional method, engine speed-dependent normalized reference levels are formed from the output signals of the knock sensors, the latter being, for example, structure-borne noise sensors. As a function of the normalized reference levels, engine speed-dependent reference threshold levels are formed, with one engine speed-dependent upper reference threshold level and one engine speed-dependent lower reference threshold level being stored as a characteristic curve in the memory of the internal combustion engine""s controller. The reference levels are formed such that they correspond to the noise level of the internal combustion engine in knock-free operation. The reference levels are set with the aid of integration of the output signals of the knock sensors, with determination of the reference levels being carried out under conditions under which no knocking occurs, for example within a predefinable measuring window.
For the purpose of the actual fault detection, in the conventional method each normalized instantaneous reference value is compared with the two engine speed-dependent reference threshold levels. In the event of a deviation, which may be predefined, of the normalized instantaneous reference level, a malfunction is detected. For example, a normalized instantaneous reference level that is below the lower reference threshold level indicates a drop-out of the knock sensor, such that no signal may proceed from it to the evaluation device. If the upper threshold level is exceeded, a malfunction of the sensor is again assumed, and appropriate replacement measures are initiated.
In order to be able to maintain a safety margin relative to the high background noise level which is caused by clatter, gas flow into the knock sensor line, etc., diagnosis is carried out only beyond a certain engine speed threshold, at which the normalized reference level is distant enough from the background noise for a reliable verification to be carried out.
The method according to the present invention and the device according to the present invention for diagnosis of knock sensors may have the advantage that the engine speed threshold for the performance of diagnosis may be set lower than in the conventional fault detection method, without the reliability of the fault detection suffering. This may provide the advantage that early diagnosis of a knock sensor fault may be made at low engine speeds, rendering it possible for appropriate replacement measures to be initiated earlier, in order to protect the internal combustion engine reliably against damage.
These advantages are obtained by providing a method or a device for fault detection having the features as described herein. The description of the present invention is based on an internal combustion engine that has only one knock sensor, which is arranged at a predefinable point on the internal combustion engine or the engine block. Several knock sensors may also be provided, and in such case specific cylinders may be assigned to each knock sensor. For fault detection or diagnosis purposes, only the signal of the loudest cylinder is used. That is, the signal of the cylinder which is loudest at the location of the knock sensor. Determination of the cylinder in question occurs in operation or when the internal combustion engine is first placed into operation, with the loudest cylinder being determined by an arrangement for maximum value selection or by comparison of the reference levels obtained for the individual cylinders.
Further advantages of the present invention may be obtained by performing fault detection or diagnosis by comparison of the instantaneous reference level of the loudest cylinder with an upper and a lower reference threshold level. The reference threshold levels may be formed as a function of the reference level of the loudest cylinder and in addition may be engine speed-dependent. The maximum value selection in operation may occur by activation of a switch-over device. For purposes of regulating the knocking, all cylinders continue to be covered.